No need to feel foolish, we often miss what to someone else may be an obvious alternative.
If you belong to the local club I am sure someone will have a spare pump lying about you could borrow.
I couldn't specify a size but am sure a small one would suffice, especially if you scooped as much duckweed out as you could by hand.
Noise from a pump can be affected by a number of factors, some easy to check are.
1. Make sure the diaphragms are in good condition with no splits.
2. Make sure the pump is sitting on a solid surface (a thin, cheap desktop will vibrate more than a solid oak one for instance, so will be a lot noisier).
3.Sit the pump on a towel, or sponge, to reduce vibration noise.
4. If you can, suspend the pump on a bit of string, so it isn't touching anything. Some pumps have a wee loop at the end you can tie the string to.